App Identity PHP API Overview

The App Identity API lets an application discover its application ID (also
called the project ID). Using
the ID, an App Engine application can assert its identity to other App Engine
Apps, Google APIs, and third-party applications and services. The
application ID can also be used to generate a URL or email address, or to make
a run-time decision.

Getting the application ID

The application ID can be found using the AppIdentityService::getApplicationId() method.

Getting the application hostname

By default, App Engine apps are served from URLs in the form
http://<your_app_id>.appspot.com, where the app ID is part of the hostname.
If an app is served from a custom domain, it may be necessary to retrieve the
entire hostname component. You can do this using the AppIdentityService::getDefaultVersionHostname() method.

Asserting identity to other App Engine apps

If you want to determine the identity of the App Engine app that is making a
request to your App Engine app, you can use the request header
X-Appengine-Inbound-Appid. This header is added to the request by the URLFetch
service and is not user modifiable, so it safely indicates the requesting
application's ID, if present.

In order for this header to be added to the request, the app making the request
must tell the URLFetch service to not follow redirects.
App Engine will then automatically add the header to the HTTP response.

In your application handler, you can check the incoming ID by reading the
X-Appengine-Inbound-Appid header and comparing it to a list of IDs allowed
to make requests.

Note: The X-Appengine-Inbound-Appid header is only set if the call is made to the appspot.com domain. If the app has a custom domain, this header will not be set.

Asserting identity to Google APIs

Google APIs use the OAuth 2.0 protocol for authentication and
authorization. The
App Identity API can create OAuth tokens that can be used to assert that the
source of a request is the application itself. The getAccessToken()
method
returns an access token for a scope, or list of scopes. This token can then be
set in the HTTP headers of a call to identify the calling application.

The following example shows how to use the App Identity API to retrieve Google Calendar contacts using OAuth.
Note: the Google API Client Libraries can also manage much of this for you automatically.

Note that the application's identity is represented by the service account name, which is typically applicationid@appspot.gserviceaccount.com. You can get the exact value by using the getServiceAccountName() method.
For services which offer ACLs, you can grant the application access by granting this account access.

Asserting identity to third-party services

The token generated by getAccessToken()
only works against Google services. However you can use the underlying signing technology to assert the identity of your application to other services. The signForApp() method
will sign bytes using a private key unique to your application, and the getPublicCertificates() method
will return certificates which can be used to validate the signature.

Note: The certificates may be rotated from time to time, and the method may
return multiple certificates. Only certificates that are currently valid are
returned; if you store signed messages you will need additional key management
in order to verify signatures later.